NOVEMBER 2021 EVENT (WED 10 NOV, 2 PM ET/11 AM PT)

WEBB'S GLIMPSE OF CHEMISTRY IN THE COSMOS – Stefanie Milam (NASA Goddard Space Flight Center)

In late 2021, scientific innovation will take the next step into space exploration and journey. The James Webb Space Telescope (sometimes called JWST or Webb), an orbiting infrared observatory that will complement and extend the discoveries of the Hubble Space Telescope but with longer wavelength coverage and greatly improved sensitivity, will launch into space on an Ariane 5 rocket from French Guiana. Webb will be the premier observatory of the next decade, serving thousands of astronomers and planetary scientist worldwide. It will study every phase in the history of our Universe, ranging from the first luminous glows after the Big Bang, to the formation of solar systems capable of supporting life on planets like Earth, to the evolution of our own Solar System. The discovery space for astrochemistry with Webb is unprecedented and will reveal new insights to planet formation, gas/solid-phase composition across the galaxy, as well as studies on the molecular inheritance of volatiles in primitive bodies in the solar system. JWST's discoveries in various objects throughout are expected to reveal the presence of previously unseen molecular gases and ices, constrain their physical state (e.g., crystalline phase and grain‐size), and measure isotopic ratios of volatile elements (H, O, C, N). In this talk, I will provide a status update of the Webb Telescope, briefly review the main science themes for JWST, and conclude with anticipated astrochemistry from JWST.

REACTION DYNAMICS OF SILANETHIONE (H₂SiS) AND THE THIOSILAFORMYL RADICAL (HSiS) VIA CROSSED MOLECULAR BEAMS – Shane Goettl (Department of Chemistry, University of Hawaii)

The investigation of the formation and reactivity of silicon- and sulfur-bearing molecules in extraterrestrial environments represents an important instrument to understand the history and chemical evolution of star forming regions, cold molecular clouds, and circumstellar envelopes. However, the fundamental reaction mechanisms by which these molecules are formed are largely unknown. Crossed molecular beam experiments represent an ideal approach to untangle the outcome of elementary gas-phase reactions leading to silicon- and sulfur-bearing molecules and their radicals of astrochemical relevance. Using this technique, we explored the bimolecular reactions of D1-silylidyne radicals (SiD; X²Π) with deuterium sulfide (D₂S) and hydrogen sulfide (H₂S), as well as the reaction of atomic silicon (Si(³P)) with thiomethanol (CH₃SH), which eventually lead to the D1-thiosilaformyl radical (DSiS) and D2- and D1-silanethione (D₂SiS and HDSiS) (SiD-D₂S/H₂S system), and the thiosilaformyl radical (HSiS) (Si-CH₃SH system). Although neither of the non-deuterated species—the thiosilaformyl radical (HSiS) and silanethione (H₂SiS) —have been observed in the interstellar medium (ISM) thus far, astrochemical models presented here predict relative abundances in the Orion Kleinmann-Low nebula to be sufficiently high enough for detection.

OCTOBER 2021 EVENT (WED 13 OCT, 2 PM ET/11 AM PT)

THE MOLECULAR BUILDING BLOCKS OF PLANETS AND OTHER STAR STUFF – Ryan Fortenberry (Department of Chemistry and Biochemistry, University of Mississippi)

The elemental abundances of rocky bodies are vastly different from the Universe at large. Our Earth is a rocky planet largely composed of oxygen followed by notable amounts of silicon, magnesium, iron, and some aluminum. However, before these atoms can form rocks which form planets, they likely must first form small molecules which can form nanocrystals and then rocks. Our work is showing that reactions leading to such objects are both highly thermodynamically and likely kinetically favorable. Additionally, this class of small, inorganic molecules has largely been unexplored in the spectroscopic literature. High-level quantum chemical quartic force field analyses are showing that these molecules often have large dipole moments and notable infrared intensities. The anharmonic vibrational frequencies of these molecules often fall beyond 9 microns and may help to explain portions of unattributed IR spectra as well as provide evidence for the early stages of nanocrystal formation in protoplanetary disks leading to planet formation. Conversely, the presence of these molecules in planetary nebulae, supernova remnants, or other stellar corpses may indicate the previous existence of rocky planets in such former planetary systems. Additionally, our work is also showing that the formation of such molecules may give clues as to the primordial environment from which they formed. Finally, other molecular families may be discussed including polycyclic aromatic hydrocarbons, prebiotic molecules, and interstellar ices.

EXTENDING THE MILLIMETER/SUBMILLIMETER ROTATIONAL SPECTRUM OF GROUND STATE PYRUVIC ACID FOR COMPARISON TO ASTRONOMICAL DATA – Connor Wright (Department of Chemistry, University of Wisconsin-Madison)

Pyruvic acid (C₃H₄O₃, or HOOCOCCH₃) is one of the main reactants in the Krebs Cycle of biological systems where sugar is metabolized and glucose is converted to lactic acid. It is also a potential interstellar precursor to the simplest chiral amino acid, alanine. Its presence in carbonaceous meteorites has already been confirmed, and its abiotic formation in experimental ice analogs implies the potential for exogenous delivery to planets via comets and asteroids. Therefore, it is important to have a complete experimental spectrum within the same wavelengths of astronomical telescopes in order to facilitate detections. Previous work on pyruvic acid reported the spectrum from 160 GHz to 314 GHz. At room temperature, these measurements only cover part of the Boltzmann peak. Additionally, with the spectral coverage now available at telescopes ranging up to and beyond 1 THz, measurements covering a significantly broader region of the molecular spectrum are required. Newly-developed experimental capabilities and sensitivities have since allowed for more sensitive measurement of the rotational transitions in this same wavelength regime and beyond. This work remeasures and extends the spectrum of pyruvic acid from 90 GHz up to 1 THz. Spectral prediction and fitting was conducted using the ERHAM program based on the recommendation of the previous work. A subsequent astronomical search for pyruvic acid was then conducted in a survey of 30 different sources. The results of both the spectroscopic fit and the astronomical search will be presented here.

CLICK HERE TO WATCH A RECORDING OF THIS EVENT.

SEPTEMBER 2021 EVENT (WED 8 SEP, 2 PM ET/11 AM PT)

THEORY AND EXPERIMENT FOR ELUCIDATING CHEMICAL EVOLUTION IN SPACE – Cristina Puzzarini (Department of Chemistry "Giacomo Ciamician", University of Bologna)

Understanding the chemical evolution of the universe is one of the main aims of Astrochemistry, with the starting point being the knowledge whether a molecule is present in the astronomical environment under consideration and, if so, its abundance. In this context, molecular spectroscopy plays the central role. The astronomical observation of the spectroscopic features of a given molecule is the definitive, unequivocal proof of its presence in the astronomical environment under consideration, with the overwhelming majority of gas-phase chemical species being discovered via their rotational signatures.
Among the goals of astrochemistry, the detection of prebiotic COMs in astrophysical environments is fundamental in view of possibly understanding the origin of life. While the evidence for molecular complexity in the universe is undisputed, there is still much to be understood about what prebiotic molecules are present and how they are formed in the typically cold and (largely) collision free environment of the interstellar medium. By means of selected examples, it will be shown that: (i) the interplay of experiment and theory in the field of rotational spectroscopy is a powerful tool in astrochemistry; (ii) state-of the-art computational approaches allow for deriving formation pathways able to explain the molecular abundances derived from astronomical observations; (iii) a rationalization of interstellar chemistry in terms of class of reactions might be a feasible way.

USING MOLECULES TO MAP PHYSICAL PARAMETERS IN ORION KL AT HIGH SPATIAL RESOLUTION – Olivia Harper Wilkins (California Institute of Technology)

Interstellar molecules are useful tools for characterizing the physical structures of dense interstellar environments. As has been illustrated repeatedly with the Orion Kleinmann-Low nebula (Orion KL), different types of molecules can be used to trace different types of environments and conditions in interstellar material. Extensive continuum and molecular line emission studies across a range of angular resolutions has revealed that Orion KL, which is the closest region of high-mass star formation to Earth, is notoriously complex and exhibits a range of physical and chemical components. However, there is still much that we do not yet know about the nebula. We conducted high angular resolution (sub-arcsecond) observations of ground-state ¹³CH₃OH and vibrationally-excited CH₃CN line emission with the Atacama Large Millimeter/submillimeter Array (ALMA) to investigate Orion KL's structure on small spatial scales (<350 au). This work provides a different perspective on Orion KL and complements previous lower angular resolution observations of the nebula. The results include temperature profiles that suggest possible sources of heating throughout Orion KL and show that each of the nebula's spatial components is not accurately described by one set of parameters. With these and past results, it is becoming increasingly apparent that Orion KL is a dynamic and heterogeneous region, and there is still much more to learn.

CLICK HERE TO WATCH A RECORDING OF THIS EVENT.

JUNE 2021 EVENT (WED 9 JUN, 2 PM ET/11 AM PT)

FORMATION OF ASTROCHEMICAL RELEVANT ORGANICS BY GAS PHASE ION-MOLECULE REACTIONS – Samy El-Shall (Mary Kapp Endowed Chair in Chemistry, Virginia Commonwealth University)

The formation of polycyclic aromatic hydrocarbons (PAHs) and polycyclic aromatic nitrogen heterocyclics (PAHNs) in solar nebulae comprises a wide range of temperatures, pressures/densities and ionizing radiation amendable for gas phase ion-molecule reactions to constitute one of the most efficient formation mechanisms. In these reactions, organics can evolve from small molecules such as acetylene (C₂H₂) and hydrogen cyanide (HCN) into ring molecular ions such as the phenyl cation (C₆H₅⁺), benzene (C₆H₆⁺•), pyridine (C₅H₅N⁺•) and pyrimidine (C₅H₄N₂⁺•) which can react further to form functional PAHs and PANHs. In this talk, we discuss results based on laboratory data from mass-selected ion mobility experiments and density functional theory calculations on the formation of covalently-bonded complex organic ions by the reactions of acetylene with the phenyl, benzene, phenylyacetylene, pyridine, pyrimidine, and benzonitrile cations. A very wide range of reaction rates has been found ranging from very slow reactions with energy barriers as in the case of benzene to reactions occurring at the collision rate as in the case of the phenyl and pyrimidine cations. Reactions of the prototypical PAH or PANH radical cations naphthalene and quinoline, respectively with the neutral molecules acetylene, benzene and pyridine have been investigated. While the reactions of the naphthalene radical cation (C₁₀H₈⁺•) with acetylene and benzene molecules result in non-covalent cluster ions, the reaction with pyridine results in the formation of covalently-bonded adducts. Interestingly, all the reactions of the quinoline radical cation (C₉H₇N⁺•) with acetylene, benzene and pyridine result in covalently bonded products. The ion-molecule reaction mechanisms observed in this work may explain formation of PANHs and more complex nitrogen rich species in nitrogen abundant areas of outer space including Titan's atmosphere.

EXPLORING SOLID-STATE CARBON ATOM CHEMISTRY: THE FORMATION OF CH₄ ICE BY C- AND H-ATOM ADDITION – Danna Qasim^* (Goddard Space Flight Center)

Methane (CH₄) ice is one of the most abundant ices to be detected in interstellar clouds. Observations of gas-phase and solid-state CH₄ in the early period of star formation, supported by astrochemical models, have shown that CH₄ is predominantly formed by grain surface reactions. Specifically, it is thought to be formed by the hydrogenation of C in the H₂O-rich ice phase of molecular clouds (A_v ~ 2). Although this pathway to CH₄ formation has been assumed for decades, it has yet to be investigated in the laboratory under controlled conditions. Previous experimental studies have been limited to the hydrogenation of graphite, as well as encountered multiple interpretations as to how CH₄ is formed in the experiment. Such limitations reflect the technical obstacles of studying carbon atom chemistry in interstellar ice analogs. In this astrocheminar, the formation of CH₄ under interstellar relevant conditions with a novel ultrahigh vacuum setup is overviewed. It is shown that CH₄ can be formed by both: the sequential hydrogenation of C and the sequential hydrogenation of C in a H₂O-rich ice, in which the latter has ~twice the formation rate. This finding supports observational surveys that CH₄ is formed by the hydrogenation of C, also when H₂O is formed. For the first time, the parameters for CH₄ formation under observationally constrained conditions are reported and can be used in astrochemical models to constrain CH₄ and CH₄ chemistry in molecular clouds, to regions where CH₄ is distributed.
^*Dr. Qasim is the 2021 recipient of the Astrochemistry Subdivision Outstanding Dissertation Award.

CLICK HERE TO WATCH A RECORDING OF THIS EVENT.

MAY 2021 EVENT (WED 19 MAY, 2 PM ET/11 AM PT)

AN AROMATIC UNIVERSE - A PHYSICAL CHEMIST PERSPECTIVE – Ralf I. Kaiser (University of Hawaii at Manoa, W. M. Keck Research Laboratory in Astrochemistry)

Polycyclic aromatic hydrocarbons (PAHs) represent key molecular building blocks leading to carbonaceous nanoparticles identified in combustion systems and extraterrestrial environments. However, the understanding of their formation and growth has remained largely elusive. Here, we present evidence based on laboratory data combined with electronic structure calculations on fundamental mass growth processes of PAHs in the gas phase via ring expansion and ring annulation. Key reaction pathways operate at ultralow temperatures such as at 10 K as present in cold molecular clods like TMC-1 synthesizing PAHs at least from two to five six membered rings such as naphthalene, anthracene, phenanthrene, triphenylene, [4]-helicene, and [5]-helicene. These elementary reactions are rapid, have no entrance barriers, and synthesize PAHs via van-der-Waals complexes and submerged barriers. This facile route to complex PAHs signifies a critical shift in the perception that PAHs can be only formed at high-temperature combustion and circumstellar conditions providing a detailed understanding of the low temperature chemistry through untangling elementary reactions on the most fundamental level. An outlook is also presented on the synthesis of PAHs in low temperature, hydrocarbon-dominated ices in deep space upon interaction with ionizing radiation; these processes are driven by non-adiabatic reaction dynamics and low-lying triplet states of acetylene leading to PAHs as complex as coronene. Overall, these mechanisms eventually lead to graphene-type PAHs and two dimensional nanostructures, providing an exciting view about the transformation of carbon in our universe.

EXPERIMENTAL STUDY OF NH₄CN AT LOW TEMPERATURES: IR OPTICAL PROPERTIES AND SUBLIMATION BEHAVIOR – Yukiko Yarnall, Reggie L. Hudson, and Perry A. Gerakines (NASA Goddard Space Flight Center)

Interstellar and cometary ices are thought to have been delivered to Earth and to have been involved in the formation of primordial biological molecules. The ESA Rosetta mission investigated comet 67P/Churyumov-Gerasimenko and identified many organic compounds, but it also detected ammonium cyanide (NH₄CN) and other salts. It is possible that ammonium salts are the parent species that produce the NH₃ and HCN gas observed in cometary comae, but further studies are required. Gas-phase interstellar HCN has been identified, NH₃ has been reported in interstellar ices, and the acid-base reaction of HCN and NH₃ under the low-temperature conditions can form NH₄CN. Thus, NH₄CN or similar ammonium-based salts may be present in interstellar ices, comets, and other planetary bodies.

We have measured several physical properties of NH₄CN at low temperatures (120-165 K), including its refractive index, density, sublimation rates, and desorption energy. Moreover, using FTIR transmission spectroscopy in the near- and mid-infrared regions, we also have determined the IR band strengths of NH₄⁺ and CN^– IR absorption features and the compound's IR optical constants (n and k) from 2 to 20 μm (5000 to 500 cm^–1). Films of pure NH₄CN were prepared at 125 K by the co-deposition of HCN with an excess of NH₃ onto an optically-flat, gold-mirror electrode located on the surface of a quartz-crystal microbalance. The refractive index and density of the resulting NH₄CN samples were determined with a double-laser interferometer system. The ice was cooled to 120 K then warmed to 165 K at 1 K min^–1 to determine the sublimation rate at each temperature, which allowed us to also measure the desorption energy of NH₄CN. IR band strengths and optical constants were determined by transmission spectroscopy of samples prepared on an IR-transparent substrate.

CLICK HERE TO WATCH A RECORDING OF PROF. KAISER'S TALK.
CLICK HERE TO WATCH A RECORDING OF DR. YARNALL'S TALK.

MARCH 2021 EVENT (WED 10 MAR, 2 PM ET/11 AM PT)

NEW FRONTIERS IN COSMIC CARBON – Brett A. McGuire (Massachusetts Institute of Technology)

The detection of the aromatic molecule benzonitrile (C₆H₅CN) in the cold, dark, starless interstellar cloud TMC-1 has opened a new window onto a previously disregarded regime of carbon chemistry at the earliest stages of star formation. Recent work by the GOTHAM collaboration has revealed that benzonitrile is only the tip of the proverbial molecular iceberg in TMC-1, hinting at a new source for interstellar polycyclic aromatic hydrocarbons (PAHs). PAHs, by some accounts, are a reservoir of as much as 25% of all interstellar carbon, making a thorough understanding of their formation and evolution essential to unraveling the tangled web of cosmic molecular evolution. Our work has also shown that aromatic carbon chemistry – including PAHs – is apparently ubiquitous throughout the early star-formation cycle and into the protostellar phase. Here, I will discuss the extent of this previously hidden reservoir of carbon and its potential implications on the chemistry of forming star and planetary systems.

ANHARMONIC FREQUENCIES FOR THE DETECTION OF LARGE MOLECULES IN SPACE – Brent Westbrook (University of Mississippi)

Quartic force fields (QFFs) offer highly accurate anharmonic rovibrational spectroscopic constants. The state-of-the-art composite approach is known as CcCR and often achieves accuracies within 1 cm^–1 of gas-phase experiment for fundamental frequencies and 20 MHz for principle rotational constants. The performance of explicitly-correlated methods is nearly as good, offering accuracies of 5-7 cm^–1 at a much lower computational cost. However, both of these QFF approaches typically rely on a complex internal coordinate system that can be difficult to derive for large or highly symmetric molecules. Extending this methodology to use generic Cartesian coordinates and the analytic derivatives found in many popular quantum chemistry packages will allow the elucidation of such spectral data for larger molecules. Preliminary work on ammonia borane and other even larger molecules demonstrates the efficacy of this new approach.

CLICK HERE TO WATCH A RECORDING OF THIS EVENT.

FEBRUARY 2021 EVENT (WED 10 FEB, 2 PM EST/11 AM PST)

THE UNSOLVED ISSUE WITH OUT-OF-PLANE BENDING FREQUENCIES FOR C=C MULTIPLY BONDED SYSTEMS– Timothy J. Lee (NASA Ames Research Center)

More than 30 years ago two groups independently identified a problem in the calculation of the out-of-plane bending (OPB) vibrational frequencies for the ethylene molecule using correlated electronic structure methods. Several studies have been done in the meantime to try and understand and resolve this issue. In so doing this problem has been found to be far more insidious than previously realized for acetylene-like and benzene-like molecules, which can become non-linear and non-planar, respectively. The one common feature that all molecules with this problem have is that they contain C=C multiple bonds, and so this has been called the "C=C multiple bond OPB frequency issue" or "the C=C OPB problem." Various explanations for this problem have been advanced such as basis set superposition error, basis set incompleteness error, linear dependences in the basis set, proper balancing of the basis set between saturation and inclusion of higher angular momentum functions, etc. and possible solutions have arisen from these suggestions. All of these proposed solutions, however, amount to one main point connecting them all: modifying the one-particle basis set in some way. None of the explanations that have been advanced, however, really fit all of the data for all of the molecules where this problem has been identified, and importantly, none of these diagnostic tests have been applied to similar molecules where this issue does not appear. In this review, the studies over the last 30 plus years are discussed and relevant data from each of these is compared and contrasted. Recent density functional theory studies on polycyclic aromatic hydrocarbons, that may or may not be connected to this issue, are also discussed as well as the implications on computing anharmonic vibrational spectra in order to compare with astronomical observations. It is hoped that by collecting and analyzing the data from these studies a path forward to understanding and resolving this issue will become evident.

THE FIRST MID-INFRARED DETECTIONS OF HNC and H¹³CN IN THE INTERSTELLAR MEDIUM – Sarah Nickerson (NASA Ames Research Center)

We present the first mid-infrared (MIR) detections of HNC and H¹³CN in the interstellar medium, and numerous HCN transitions. Our observations span 12.8 to 22.9 μm towards the hot core Orion IRc2, obtained with the Echelon-Cross-Echelle Spectrograph aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA/EXES). 5 km/s resolution distinguishes individual rovibrational transitions of the three molecules, allowing direct measurement of their excitation temperatures, column densities, and relative abundances. HNC and HM^13MCN share temperatures of 100 K with a local standard of rest velocity of -7 km/s. HCN shows two velocity components at -7 km/s at 165 K, and 1 km/s at 309 K. The -7 km/s velocity component measured for all three molecules is similar to an outflow from the nearby high mass protostar Radio Source I, and are likely associated with it. The 1 km/s component is the hottest measured HCN to date towards IRc2 and closest to the hot core’s centre. EXES's smaller beam size compared to most other detections allows us to focus on the hot core itself without confusion from surrounding sources. Previous observations at longer wavelengths detected colder components of these three molecules in emission, while the MIR observations are hotter and in absorption. We utilize a gas-grain chemical network to model the HCN/HNC evolution, which reaches our derived HCN/HNC=72 after 10⁶ years. This is much older than the region's explosive event 500 years ago, suggesting that the hot core's origins predate this event. Our derived ¹²C/¹³C=13 is lower than measurements at longer wavelengths. Several other recent observations towards star-forming regions also show similarly unexpectedly low isotope ratios. This points to the possibility that the isotope chemistry in these regions is not yet fully understood.

CLICK HERE TO WATCH A RECORDING OF THIS EVENT.

JANUARY 2021 EVENT (WED 13 JAN, 2 PM EST/11 AM PST)

COMPUTATIONAL EXPLORATIONS OF SOME NOVEL GROWTH MECHANISMS LEADING TO PAH AND PANH SPECIES – Martin Head-Gordon (Kenneth S. Pitzer Distinguished Professor of Chemistry, University of California, Berkeley)

There have been significant improvements in the ability of density functional theory (DFT) methods to accurately model chemical reactions over the past 5 years. These improvements will be summarized, and the resulting methods applied to address some interesting pathways towards the formation and growth of aromatics under conditions relevant to the interstellar medium. The reactions considered will include viable routes to formation of benzene from acetylene clusters or ices under ionizing conditions, and the corresponding pathways to pyridine (with relevance to the origin of DNA bases) in the presence of both acetylene and hydrogen cyanide. If time permits, ionization-induced bond formation between aromatics and pyridine will also be considered to unravel the nature of this exotic strong interaction.

PRESTELLAR PROVENANCE OF COMET 67P/CHURYUMOV-GERASIMENKO – Maria Drozdovskaya (Center for Space and Habitability, University of Bern)

Our Solar System harbors several thousand known comets with many more awaiting discovery. Comets are thought to be some of the most pristine relics that survive to this day, which may shed light on the volatiles and refractories that nourished our infant Solar System. In my talk, I will discuss the results from the ESA Rosetta mission that were obtained during its two year-long escort of comet 67P/Churyumov-Gerasimenko. I will address the story told by the comet's volatile inventory and deuteration, as deduced by the ROSINA instrument. The connections with star-forming regions, such as the low-mass IRAS 16293-2422, will be explored. In my talk, I will present evidence that leads to the conclusion that comets are capable of revealing the full evolutionary scenario of the low-mass system that we call home.

CLICK HERE TO WATCH A RECORDING OF THIS EVENT.

DECEMBER 2020 EVENT (WED 9 DEC, 2 PM EST/11 AM PST)

THE UNEXPECTED CHEMISTRY IN PLANETARY NEBULAE: FROM CO TO C₆₀ – Lucy M. Ziurys (Regents Professory, CBC and Astronomy, University of Arizona)

For decades, planetary nebulae (PNe) have been considered to have limited molecular content, confined mostly to diatomic molecules. The strong ultraviolet radiation field generated by the central white dwarf star was thought to destroy molecules readily generated in the prior Asymptotic Giant Branch (AGB) phase. Theoretical calculations verified this line of thought. Over the past several years, however, we have been conducting observations of various molecules in PNe at millimeter wavelengths. We have now detected polyatomic molecules such as HCN and HCO⁺ in over 30 PNe, and species as complicated as CH₃CN, H₂CO, and c-C₃H₂ in several select nebulae. Furthermore, the molecular abundances do not appear to significantly vary with the age of the nebulae over the 10,000 year PN lifetime. Circumstellar abundances appear to be principally altered in the protoplanetary nebulae (PPNe) stage. Such molecular material must seed diffuse clouds, accounting for the polyatomic species observed there. Notably, C₆₀ is also observed in some PNe. We have also conducted recent solid-state laboratory imaging and spectroscopy, as well as molecular dynamics (MD) simulations, that suggest that C₆₀ is formed in the PPNe phase from the destruction of silicon carbide (SiC) grains.

ASTROCHEMICAL FORECASTING WITH MACHINE LEARNING – Kelvin Lee, (Department of Chemistry, Massachusetts Institute of Technology)

Since the first molecules were detected in space, we have now reached a point where chemical and physical complexity in the interstellar medium reaches the boundaries of what human expertise and intuition alone can achieve. With every new molecule we discover, the question "What comes next?" grows more and more difficult to answer as more possibilities emerge. Conventionally, we turn to chemical models for guidance; this may be complicated when considering complex, non-LTE processes such as shocks, radiation, and grain-surface chemistry. Moreover, expansion of chemical networks typically requires hand-picked reactions and species, requiring an exhaustive knowledge of chemical and astrophysical literature, and can impose human bias on which reactions and molecules are important. As a complimentary approach to conventional chemical models, we have developed an unsupervised machine learning pipeline for predicting molecular abundances in a non-parametric fashion. Leveraging tools originally developed in high throughput drug discovery and data science, our pipeline captures and uses millions of molecules from various databases to create chemically descriptive vector representations for quantitative comparison. These representations are subsequently used to predict molecular properties in a given environment; as a proof-of-concept, we use the well-characterized chemical inventory of TMC-1, including the latest discoveries from the GOTHAM collaboration. We show that the model can be successfully conditioned on an inventory, able to reproduce column densities of unseen molecules to within an order of magnitude without any tuning parameters. Simultaneously, we are able to use the model to predict column densities of hundreds of thousands of molecules not yet detected in space, as a way to guide efforts, as well as provide a robust statistical baseline for expected abundances.

CLICK HERE TO WATCH A RECORDING OF THIS EVENT.

NOVEMBER 2020 EVENT (WED 11 NOV, 2 PM EST/11 AM PST)

PREBIOTIC ASTROCHEMISTRY IN THE "THz-GAP" – Susanna Widicus Weaver (Department of Chemistry, University of Wisconsin - Madison)

Small reactive organic molecules are key intermediates in interstellar chemistry, leading to the formation of biologically-relevant species as stars and planets form. These molecules are identified in space via their pure rotational spectral fingerprints in the far-IR or terahertz (THz) regime. Despite their fundamental roles in the formation of life, many of these molecules have not been spectroscopically characterized in the laboratory, and therefore cannot be studied via observational astronomy. The reason for this lack of fundamental laboratory information is the challenge of spectroscopy in the THz regime combined with the challenge of studying unstable molecules. Our laboratory research involves characterization of astrophysically-relevant unstable species, including small radicals that are the products of photolysis reactions, organic ions formed via plasma discharges, and small reactive organics that form via O(¹D) insertion reactions. Our observational astronomy research seeks to examine the chemical mechanisms at play in a range of interstellar environments and to identify chemical tracers that can be used as clocks for the star-formation process. In this seminar, I will present recent results from our laboratory and observational studies that examine prebiotic chemistry in the interstellar medium. I will discuss these results in the broader context of my integrative research program that encompasses laboratory spectroscopy, observational astronomy, and astrochemical modeling.

PHYSICOCHEMICAL MODELS: SOURCE-TAILORED OR GENERIC? – Beatrice Kulterer, (Center for Space and Habitability, Universität Bern)

Physicochemical models can be powerful tools to trace the chemical evolution of a protostellar system and allow to constrain its physical conditions at formation. I will discuss whether source-tailored modelling is needed to explain the observed molecular abundances around young, low-mass protostars or if, and to what extent, generic models can improve our understanding of the chemistry in the earliest stages of star formation (Kulterer et al. 2020). The physical conditions and the abundances of nine simple and abundant molecules based on three models are compared. The physical models considered are 1D or 2D, the chemical networks consider two or three phases. After establishing the discrepancies between the calculated chemical output, the calculations are redone with the same chemical model for all three sets of physical input parameters. With the differences arising from the chemical models eliminated, the output is compared based on the influence of the physical model. Results suggest that the impact of the chemical model is small compared to the influence of the physical conditions, with considered timescales having the most drastic effect. Source-tailored models may be simpler by design; however, likely do not sufficiently constrain the physical and chemical parameters within the global picture of star-forming regions. Generic models with more comprehensive physics may not provide the optimal match to observations of a particular protostellar system, but allow a source to be studied in perspective of other star-forming regions.

CLICK HERE TO WATCH A RECORDING OF THIS EVENT.

OCTOBER 2020 EVENT (WED 14 OCT, 2 PM EDT/11 AM PDT)

THE RIDDLE OF COMPLEX ORGANIC MOLECULES – Eric Herbst (Departments of Chemistry and Astronomy, University of Virginia)

In addition to stars, galaxies such as our own Milky Way contain interstellar matter, much of which is condensed into so-called interstellar clouds consisting of gas and nanoparticles known as dust grains. The clouds, ranging in size from a few to 100's of light years in extent, are of great interest as the ultimate sites of star and planetary formation. The denser clouds contain large numbers of molecules in the gas phase, mainly organic in nature, divided into classes of molecules dependent upon the age and physical nature of the clouds. Molecules are also observed in ice mantles of cold dust particles. Two distinctive classes of gaseous molecules are known as "carbon chains" and "complex organic molecules (COMs)". Carbon chains are exotic, very unsaturated, and often linear, whereas COMs resemble small terrestrial organic solvents, consisting of amines, alcohols, esters, etc. Until recently, based on observations, it was thought that carbon chains exist solely in cold dense clouds, whereas COMs exist in warmer regions in which star formation is occurring, known as "hot cores." More recently, COMs have been discovered in cold regions as well, introducing more complexity into our understanding of their chemistry. This talk will be concerned mainly with the local build-up chemistry thought to form COMs in both types of sources, and the degree of success that has been achieved.

FORMATION OF COMPLEX ORGANIC MOLECULES IN THE TRANSLUCENT CLOUD VIA TOP-DOWN PROCESSING ON DUST GRAINS – Ko-Ju Chuang,^* (Max Planck Institute for Astronomy)

Interstellar complex organic molecules (COMs) have been identified toward various star-forming regions from translucent clouds to the solar system. Interstellar sugar-like species (C_nH_2nO_n) have been intensively studied along with the bottom-up approaches; the ice chemistry scheme of CO-H₂CO-CH₃OH through "energetic" and/or "non-energetic" processing on dust grains. However, the icy origin of acetaldehyde and its (de-)hydrogenated derivatives (C₂H_nO), which are often observed in molecular clouds before CO freezes out, remains unclear. In this talk, I will present the laboratory study on the solid-state reactions that involve C₂H₂, which is one of the common hydrocarbon fragments of PAHs or hydrogenated carbonaceous dust (HAC) in the top-down scenario, and H/OH-radicals along with the H₂O formation/destruction sequence on grain surfaces under molecular cloud conditions. It is concluded that C₂H₂ readily acts as a molecular backbone providing a solid-state route for the formation of COMs, such as ketene (CH₂CO), acetaldehyde (CH₃CHO), vinyl alcohol (CH₂CHOH), ethanol (CH₃CH₂OH), and possibly acetic acid (CH₃COOH). The reaction network linking the above complex species described by the formula C₂H_nO is present.
^*Dr. Chuang received Honorable Mention in the 2020 competition for the Astrochemistry Subdivision Outstanding Dissertation Award.

CLICK HERE TO WATCH A RECORDING OF THIS EVENT.

SEPTEMBER 2020 EVENT (WED 9 SEP, 2 PM EDT/11 AM PDT)

THE MOLECULAR UNIVERSE – Alexander Tielens (Leiden Observatory, Leiden University and Astronomy Department, University of Maryland)

Over the last 20 years, we have discovered that we live in a molecular Universe: a Universe with a rich and varied organic inventory; a Universe where molecules are abundant and widespread; a Universe where molecules play a central role in key processes that dominate the structure and evolution of galaxies; a Universe where molecules provide convenient thermometers and barometers to probe local physical conditions. Understanding the origin and evolution of interstellar and circumstellar molecules is therefore key to understanding the Universe around us and our place in it and has therefore become a fundamental goal of modern astrophysics. The field is heavily driven by new observational tools that have become available over the last 20 years; in particular, space-based missions that have opened up the IR and submillimeter window at an ever-accelerating pace. Furthermore, our progress in understanding the Molecular Universe is greatly aided by close collaborations between astronomers, molecular physicists, astrochemists, spectroscopists, and physical chemists who work together in loosely organized networks. In this talk, I will sketch the progress that we have made over the last 20 years and outline some of the challenges that are facing us. The focus will be on understanding the unique and complex organic inventory of regions of star and planet formation that may well represent the prebiotic roots to life.

A PHOTOIONIZATION REFLECTRON TIME-OF-FLIGHT INVESTIGATION OF PHOSPHORUS CHEMISTRY IN EXTRATERRESTRIAL ICES – Andrew M. Turner,^* Cornelia Meinert, Ralf I. Kaiser (University of Hawaii at Manoa)

Multiple phosphorus-containing compounds have been detected in the Solar System (planetary atmospheres, comets, meteorites) along with interstellar and circumstellar environments. Of particular astrobiological interest are alkyl phosphonic acids (RH₂PO₃, R = methyl, ethyl, propyl, and butyl) extracted from the Murchison meteorite. These phosphonic acids are the only extraterrestrial phosphorus-containing organic compounds thus far discovered and offer a bioavailable and highly soluble form of phosphorus. This project investigates the synthesis of phosphorus-containing products of electron-irradiated interstellar ice analogues containing phosphine (PH₃), water (H₂O), carbon dioxide (CO₂), and hydrocarbons such as methane (CH₄). Phosphine is known to exist in circumstellar envelopes (IRC +10216), has been considered for comets (67P/Churyumov-Gerasimenko), and may serve as the phosphorus source of complex organic compounds such as the alkyl phosphonic acids. Utilizing in situ analysis techniques such as quadrupole mass spectrometry (QMS), tunable-photoionization reflectron time-of-flight mass spectrometry (PI-ReTOF-MS), and infrared spectroscopy (FTIR) in addition to ex situ analysis by secondary-ion mass spectrometry (SIMS) and two-dimensional gas chromatography mass spectrometry (GCxGC-TOF-MS), the intermediates and products of these irradiated ice analogues are characterized to demonstrate the potential to synthesize organic phosphine-containing molecules in astrophysical environments. Notable results include phosphanes (P_xH_x+2), methylphosphanes (CH₃P_xH_x+1), and phosphorus oxoacids (H₃PO_x, x=1–4, and pyrophosphoric acid (H₄P₂O₇) along with their alkylated equivalents such as prebiotically significant methylphosphonic acid (CH₃P(O)(OH)₂) and methylphosphate (CH₃OP(O)(OH)₂).
^*Dr. Turner is the 2020 recipient of the Astrochemistry Subdivision Outstanding Dissertation Award.

CLICK HERE TO WATCH A RECORDING OF THIS EVENT.

ZOOM INFORMATION. Registration is required to attend all Astrocheminars via Zoom. A confirmation email with the link for the event will be sent to the email account used for your Zoom account. Zoom attendance limited to 300 people. A Zoom account is required for all AstroCheminars. To create a free Zoom account, visit the Zoom website. After you create an account, download and install the Zoom client. (A link to download the Zoom client may be found under RESOURCES on the Zoom webpage.).

The Astrochemistry Subdivision of the American Chemical Society sponsors a monthly seminar series that features the work of astrochemists from around the world, including senior experts in the field, postdocs, and students. The seminar series will feature both invited and contributed talks. To submit an abstract to be considered as a 15-minute contributed talk for a future event, please complete this form or email acs.astrochem@gmail.com.

AstroCheminars are open to everyone. Membership in the Subdivision is not required.